In December 2015, Juniper Networks announced multiple security vulnerabilities stemming from unauthorized code in ScreenOS, the operating system for their NetScreen Virtual Private Network (VPN) routers. The more sophisticated of these vulnerabilities was a passive VPN decryption capability, enabled by a change to one of the parameters used by the Dual Elliptic Curve (EC) pseudorandom number generator.

In this paper, we described the results of a full independent analysis of the ScreenOS randomness and VPN key establishment protocol subsystems, which we carried out in response to this incident. While Dual EC is known to be insecure against an attacker who can choose the elliptic curve parameters, Juniper had claimed in 2013 that ScreenOS included countermeasures against this type of attack. We find that, contrary to Juniper's public statements, the ScreenOS VPN implementation has been vulnerable to passive exploitation by an attacker who selects the Dual EC curve point since 2008. This vulnerability arises due to flaws in Juniper's countermeasures as well as a cluster of changes that were all introduced concurrently with the inclusion of Dual EC in a single 2008 release. We demonstrate the vulnerability on a real NetScreen device by modifying the firmware to install our own parameters, and we show that it is possible to passively decrypt an individual VPN session in isolation without observing any other network traffic. This incident is an important example of how guidelines for random number generation, engineering, and validation can fail in practice. Additionally, it casts further doubt on the practicality of designing a safe "exceptional access" or "key escrow" scheme of the type contemplated by law enforcement agencies in the United States and elsewhere.

1. Introduction

In December 2015, Juniper announced that an "internal code review" revealed the presence of "unauthorized code in ScreenOS that could allow a knowledgeable attacker [...] to decrypt VPN connections." In response to this, Juniper released patched versions of ScreenOS, the operating system powering the affected NetScreen devices, but has declined to disclose any further information about the intrusion and vulnerability.

Immediately following Juniper's advisory, security researchers around the world—including our team—began examining the ScreenOS firmware to find the vulnerabilities Juniper had patched. They found that the change that rendered ScreenOS encryption breakable did nothing but replace a few embedded constants in Juniper's pseudorandom number generator. The reason why this results in an attacker being able to decrypt connections is Juniper's design decision to use the NSA-designed Dual EC Pseudorandom Number Generator (PRNG).4,12 Dual EC has the problematic property that an attacker who knows the discrete logarithm of one of the input parameters (Q) with respect to a generator point, and is able to observe a small number of consecutive bytes from the PRNG, can then compute the internal state of the generator and thus predict all future output. Thus, it is critical that the discrete logarithm of Q remain unknown. The changes to the ScreenOS code replaced Juniper's chosen Q with one selected by the attacker.

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